1. Field of the Invention
This invention generally relates to the measurement of temperatures and more particularly relates to heat shields for aspirating or suction pyrometers used in the measurement of high gas temperatures such as those encountered in glass melting furnaces.
2. Description of the Prior Art
It is desirable in the glass making industry to measure accurately the temperatures of the gases at different locations in the overall system of producing glass, particularly the gas temperatures in a melting furnace occuring at its ports, regenerators tunnels, flues, chimney and ejectors. Measured under actual operating conditions, the temperatures serve as bases for improvements in operations, modifications in design, and fuel and power savings. As a result, longer furnace life, higher tonnages, improved quality, and lower costs for the production of glass may be achieved.
In a conventional glass melting tank or furnace, fuel is alternately fired, using preheated combustion air, from one side and then the other through a series of ports along each side of the tank at right angles to the flow of molten glass. The raw materials are continually fed at one end of the tank and molten glass is removed from its other end. The variations and conditions at the various ports down each side of the tank are therefore important in determining the variations in temperature undergone by the raw materials during melting and the glass after melting.
In such glass melting furnace systems, the temperature of the combination air and exhaust gases may be substantially different from the temperature of the surrounding bodies, and the heat exchanged by radiation between the bodies and the temperature measuring instrument may predominate over that exchanged by convection. The exchange of heat by radiation from or to the adjacent bodies can influence the instrument reading so that it may indicate the temperature of such bodies or some temperature intermediate that of the bodies and the combustion air or exhaust gases rather than the true temperature of these gases.
Generally, in the above-mentioned environment a sheathed thermocouple is employed to measure the temperature of a gas. As is known, a thermocouple indicates its own temperature, and, if it is to determine that of a gas, its hot junction must attain the temperature of the gas. In the case of the sheathed thermocouple, the surface of the sheath receives heat from gas by convection. This heat then passes through the sheath to the hot junction of the thermocouple, but at the same time the sheath exchanges heat by radiation with the surrounding bodies and loses heat by conduction therealong. Hence, the temperature reached by the hot junction of the thermocouple may be different from the true temperature of the gas, particularly when temperatures above 1400.degree. C. (2550.degree. F.) are encountered.
Thus, in order to measure accurately temperatures of hot gases whose temperatures are different from those of their surroundings, aspirating pyrometers are conventionally employed. As is known, as aspirating pyrometer is an instrument wherein the convective heat transfer to a sheathed thermocouple from a gas is increased by drawing the gas over it at high velocity and at the same time shielding the thermocouple from heat radiating to or from the surrounding bodies so that the temperature of the hot junction of the thermocouple will be substantially the same as the temperature of the gas it is sensing.
In the past it has been proposed to construct a heat shield from standard thin wall refractory tubing. Generally speaking, this type of shield construction comprises a large diameter tube housing a series of small diameter tubes which are circumferentially arranged around and bonded to the inside surface of the larger tube.
In U.S. Pat. No. 4,038,105 it is illustrated a radiation shield for a pyrometer that comprises, as illustrated in FIG. 1 therein, a first tube having arranged circumferentially on the inside smaller tube for gas flow. This radiation shield is surface coated with a silicon carbide or other castable cement to provide protection against thermal shock and to seal the point of contact between the pyrometer shield and the cooled probe. The radiation shields are subject to great thermal shock as they are inserted into the furnace and withdrawn into ambient temperatures for insertion at a different point in the furnace.
There is a continuing problem with thermal shock breakage of the probe radiation shields which results in a loss of calibration, possible loss of the thermocouple operability, loss of time in replacement of the thermocouple and radiation shield and the costly loss of the radiation shields themselves. Therefore, there is a continuing need for a radiation shield possessing high resistance to thermal shock giving a long life. Further there is a need for a radiation shield of which will be less costly as it does not often have to be replaced necessitating both materials and labor costs. There is a need for a long life radiation shield that can be utilized for measuring temperatures up to 3200.degree. F. and withdrawn into ambient temperature repeatedly.